I'm looking for an electronically controlled air valve solution which would provide a way to mix external air into a closed loop air circulation system used for climate control in an enclosure. The enclosure box will have an input and output duct (1 inch PVC for example), with several external components connected in series along the air circulation loop, including an inline variable rate fan to circulate the air, and additional inputs such as a heater, humidifier, etc. These different inputs will be controlled with a micro-controller with PID algorithms to maintain temperature and humidity in the enclosure.

The Air Mixer stage should be able to mix a variable amount of external air into the system, an air exchanger really since it would have an exhaust and intake. It is this part of the system which I am working on a solution for.

One thought I had was to have 2 valves, one before the fan and one after, which are connected to the external air. Opening these valves, I believe should lead to the one before the fan being the intake and the one after being the exhaust. I'm not sure what kinds of physics this involves though (fluid dynamics?) and would also like to make this mixture amount variable if possible, but could resort to on/off modulation if necessary. It seems like varying the amount of pressure difference between these two points in the system would result in more or less external air being pulled into the system and thus the internal air being exhausted.

Edit: After more searching I found several "ball valves" which look like they can be used as variable flow valves, by activating the motor for only a certain time or with a position sensor. I think I could make one of these an exhaust valve (or maybe intake?) and the other a fixed open/close valve to provide some variability and a complete "closed loop" option.

Any tips on how to easily provide a means of variable exchange of external air with a closed loop circulation system would be greatly appreciated!

Airflow Diagram

  • $\begingroup$ If you don't need to have it sealed closed, it may be easier to have 2 fans (in & out, or dry-air-in & humid-air-in), and no valves. Fans can give you good flow but can only produce very low pressure. $\endgroup$
    – Pete W
    Sep 13 at 21:59
  • $\begingroup$ @PeteW I do want to keep it sealed well to maintain conditions when the target values are reached and vent to external air only to bring humidity down (dry air). The application is for culturing/curing meat in a refrigerator in my garage. The refrigerator will be used for cooling, a resistive heating element for heating, and an ultrasonic mister for humidity - throughout the seasons. I like the idea of having a lot of those inputs be external on an air circulation loop rather than cramming them in the fridge. Air flow circulation can be slow with minimal pressure involved. $\endgroup$ Sep 14 at 3:40
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    $\begingroup$ Adding external air at its simplest is an economizer HVAC system. Inlet must be before the fan, outlet after the fan. The outlet could be just a flapper valve. $\endgroup$
    – Tiger Guy
    Sep 14 at 12:38
  • $\begingroup$ @TigerGuy Thanks for the ideas. Good point about the outlet potentially just being a flapper valve. $\endgroup$ Sep 15 at 19:36

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